When the operation of an electronic circuit or component in which electronic devices are densely packed such as on an LSI (Large Scale Integrated circuit) and an LSI package is verified, an electromagnetic field measuring apparatus is used to measure an electric field and a magnetic field in the vicinity of the circuit or component. The electromagnetic field measuring apparatus measures an electric field or a magnetic field in the vicinity of an object to be measured by using a laser beam and an electro-optic (EO) material or a magneto-optic (MO) material. Examples of the related art of this kind include apparatuses disclosed in Non-patent literatures 1, 2 and 3. These literatures disclose reports on evaluation results of microwave circuits and antennas obtained by using electromagnetic field measuring apparatuses. The electromagnetic field measuring apparatus includes an optical measuring device including a laser light source, and an electromagnetic field sensor made of EO/MO material. In the electromagnetic field measuring apparatus, a laser beam is launched into the EO/MO material (electromagnetic field sensor) and changes in the refractive index of the EO/MO material according to the nearby electromagnetic field strength are detected. By doing so, an electric field and a magnetic field are measured.
Further, the electromagnetic field measuring apparatus of this kind includes, in addition to the configuration in which light emitted from a laser light source propagates through the space and enters the EO/MO material, another configuration in which the optical measuring device and the EO/MO material are entirely connected through an optical fiber(s) so that a laser beam propagates through the optical fiber and enters the EO/MO material. In the electromagnetic field measuring apparatus, it is possible to carry out a measurement with a high spatial resolution in a minuscule area by carrying out micromachining on the EO/MO material. Therefore, it has been expected that the electromagnetic field measuring apparatus will exhibit its ability in performance evaluations, fault diagnoses, or electric designing of densely-packed electronic circuits and components.
For example, as shown in FIG. 11, an electromagnetic field measuring apparatus disclosed in Non-patent literature 1 includes a laser light source 1, an optical fiber 2, a light amplifier 3, an optical fiber 4, a polarization controller 5, an optical fiber 6, an optical circulator 7, an optical fiber 8, an EO/MO crystal 9, an optical fiber 10, an analyzer 11, an optical fiber 12, a light amplifier 13, an optical fiber 14, a optical receiver 15, and an RF (Radio Frequency) spectrum analyzer 16. Each of the light amplifiers 3 and 13 is composed of, for example, an EDFA (Erbium-Doped Fiber Amplifier). Further, a circuit to be measured TS, i.e., a circuit whose operation is to be verified, is disposed near the above-mentioned EO/MO crystal 9.
In this electromagnetic field measuring apparatus, carrier (carrier wave) signal light pa having a frequency fOF is emitted from the laser light source 1. The carrier signal light pa enters the light amplifier 3 through the optical fiber 2. Then, the carrier signal light pa is amplified in the light amplifier 3 and emitted as signal light pb. The signal light pb enters the polarization controller 5 through the optical fiber 4. Then, the signal light pb is adjusted in polarization in the polarization controller 5 and emitted as signal light pc. The signal light pc propagates through the optical fiber 6, the optical circulator 7, and the optical fiber 8, enters the EO/MO crystal 9 as signal light pd, and returns to the optical fiber 8 again as signal light pe. In this point, the signal light pe is modulated due to the electromagnetic field (frequency fRF) caused by the circuit to be measured TS.
As a result of this modulation, the signal light pe has such a spectrum that sideband peaks (an upper-side wave having a frequency [fOF+fRF] and a lower-side wave having a frequency [fOF−fRF] appear on both sides of the carrier signal (frequency fOF). The signal light pe enters the analyzer 11 through the optical fiber 8, the optical circulator 7, and the optical fiber 10, and is emitted as signal light pg. The signal light pg enters the light amplifier 13 through the optical fiber 12. Then, the signal light pg is amplified in the light amplifier 13 in order to improve the sensitivity to the optical receiver 15 and emitted as signal light ph. In this process, when the level of the carrier signal is high, the amplification factor of the sideband becomes lower because of the saturation characteristic of the light amplifier 13 (that is, when the level of the carrier signal is high, the sideband cannot be sufficiently amplified). Therefore, the level of the carrier signal is attenuated to some extent by the analyzer 11 according to the saturation characteristic of the light amplifier 13. The signal light ph is input to the optical receiver 15 and converted into an electric signal ed. This electric signal ed has a frequency fRF. The electric signal ed is input to the RF spectrum analyzer 16, and the spectrum of the electromagnetic field (frequency fRF) caused by the circuit to be measured TS is analyzed.
Further, a magnetic field measuring apparatus disclosed in Non-patent literature 2 has a similar configuration and operates in a similar manner to the above-described electromagnetic field measuring apparatus disclosed in Non-patent literature 1, though it is specialized to the measurement of magnetic fields.
Further, in an electromagnetic field measuring apparatus disclosed in Non-patent literature 3, a light modulator is introduced and the frequencies of electromagnetic waves to be measured are down-converted by this light modulator.